Electron magnetic resonance (EMR) spectrometers, also known as electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectrometers, provide an indication of the chemical environment of electrons in a material by measuring the absorption of electromagnetic radiation, typically at microwave wavelengths, by the material when the material is placed in a magnetic field. An absorption measurement is made by sweeping the frequency of the incident electromagnetic radiation, by sweeping the strength of the magnetic field, or by pulsing the incident electromagnetic radiation and measuring the transient response of the sample.
Presently available EMR spectrometers operate generally at incident electromagnetic frequencies of 94 GHz or less. When higher frequencies are employed, the required reduction in waveguide size results in higher electromagnetic transmission losses and increased difficulties in fabricating the sample cavity due to the relative tolerances for smaller devices. To overcome these problems high frequency EMR spectrometers have been designed which include a single adjustable dimension semiconfocal Fabry-Perot interferometer as a sample cavity. However, such cavities do not permit the coupling of radiation into the sample cavity to be adjusted.
Because of this inability to vary the coupling of radiation into the sample cavity, the "finesse" or cavity amplification (a function of the ratio of power stored in the cavity to incident power) is reduced in order to accommodate a wide range of sample characteristics. That is, the overall Q of the system is given by the expression: ##EQU1## where Q.sub.unloaded is the Q of the cavity without the sample and Q.sub.sample is the contribution to Q.sub.overall by the sample. Although it is desirable to have a large Q.sub.overall, the dominant term in present devices is the 1/Q.sub.unloaded, making the contribution due to Q.sub.sample typically small. This reduction in finesse results in a reduction in spectrometer sensitivity.
The present invention avoids these difficulties in high frequency EMR spectrometers.